In general, as a drive control method for a brushless motor used in a starter of an internal combustion engine, there is known a sensor type drive control circuit in which a plurality of Hall elements are implemented around a rotor, for detecting a position of the rotor (permanent magnet side) within the brushless motor. However, in this sensor type drive control circuit, a plurality of Hall elements need to be implemented and a position detection magnet or the like needs to be implemented separately from the rotor as necessary. Consequently, it has been an obstacle to reduction in size and reduction in cost. Moreover, a variation occurs in position detection accuracy due to the condition of the Hall element installation. Consequently, there has been a strong demand for realizing a sensorless type drive control circuit that detects a rotor position without the use of a sensor such as Hall element, and this has been realized at present.
In the conventional sensorless type drive control of a brushless motor, there is known a drive control method based on 120° conduction (a method in which electric power is conducted only during the 120° period of the entire 180° phase period) in which: a drive electric current is caused to flow to an armature coil at a high speed timing that does not cause the motor to rotate; then from the drive current rise characteristic thereof, a rotor stop position within the brushless motor is detected; a conduction start phase is then determined and the rotor is rotated; and having started conduction, a zero-cross point of the phase voltage of a non-conduction phase is detected, to thereby detect a rotor position. As other conduction method, there is the 180° conduction method in which a rotor position is obtained based on rotor position detection waveforms detected from a sub coil, and 180° conduction is performed to thereby perform drive control of a brushless motor (for example, refer to Patent documents 1, 2, 3, 4, and 5, and Non-patent documents 1 and 2).
The above detection of a rotor stop position when the motor is in the stop state is such that electric power is conducted to phases U, V, and W for a short period of time (for example, several msec), and the difference in rising electric current associated with the relationship between the position of (the magnet of) the rotor and the position of the motor coil, is taken to thereby detect a rotor stop position.
This rotor stop position detection method is such that where the L (inductance) values of coils of the respective phases U, V, and W wound respectively on multiple poles of a three phase motor are equal respectively in the phases U, V, and W, it utilizes the fact that a difference occurs in the magnetic field received on the coils of the respective phases according to the position of (the magnet of) the rotor, and consequently a difference occurs in the electric current rise when electric power is conducted to the coil of the same L value.    [Patent document 1] Specification of Japanese Patent No. 3673964    [Patent document 2] Japanese Unexamined Patent Application, First Publication No. 2006-81396    [Patent document 3] Japanese Unexamined Patent Application, First Publication No. H10-257792    [Patent document 4] Japanese Unexamined Patent Application, First Publication No. H07-274585    [Patent document 5] Japanese Unexamined Patent Application, First Publication No. 2001-327185    [Non-patent document 1] Shun-ichi Kondo, “Brushless DC Motor Control Circuit Design”, Transistor Technology, CQ Publishing Co. Ltd., pp. 212-220, February, 2000    [Non-patent document 2] Satoshi Kusaka, “Brushless DC Motor Driving Method”, Transistor Technology, CQ Publishing Co. Ltd., pp. 221-228, February, 2000